Sulfide is an unwanted component of many waste streams. It can occur naturally or as the result of industrial processes. It also is generated by the absorption of hydrogen sulfide in an aqueous liquid as is common in scrubber systems, particularly alkaline scrubbers. If the scrubber liquid is alkaline a portion of the hydrogen sulfide will be converted to dissolved bisulfide (HS−) and sulfide (S−2). The proportion of these species depends on the pH of the liquid. The term sulfide used in this invention includes all forms of inorganic sulfide including hydrogen sulfide, bisulfide ions, sulfide ions and polysulfide ions. If alkaline sulfide-bearing solutions are neutralized or acidified, soluble sulfides are converted to hydrogen sulfide, potentially off-gassing from the liquid. Hydrogen sulfide gas is malodorous and toxic. Liquids that contain sufficiently high levels of sulfide are classified by US EPA regulations as reactive hazardous wastes because of their potential to generate hydrogen sulfide when acidified. It is often desirable to remove or destroy sulfides present in aqueous fluids. One method of eliminating sulfides is to oxidize them to a new compound that is not malodorous or toxic.
Oxidation of sulfides in aqueous liquids can be accomplished chemically with oxidizing agents such as hydrogen peroxide, chlorine dioxide, hypochlorite salts, methylmorpholine-N-oxide, or nitrate/nitrites. These methods are effective but have drawbacks which can include high chemical costs, handling of hazardous chemicals and formation of unwanted by-products. Oxidation can be accomplished biologically, but this is usually expensive and can produce odors in the treatment units. A third method is oxidation with molecular oxygen in the presence of a catalyst. Sulfides may also be treated by other methods such as absorption or sequestering.
The most common catalyst for sulfide oxidation is a chelated metal catalyst, most particularly iron chelated by an aminopolycarboxylic acid. The normal product of oxidation with this catalyst in aqueous fluids is elemental sulfur which precipitates. The catalyst is typically regenerated with molecular oxygen, normally atmospheric air which can also degrade the catalyst. This method for oxidizing sulfides is not without drawbacks. It requires removal of solid elemental sulfur and replenishment of catalyst.